1. Field of the Invention
The present invention relates to novel colloidal organic sols comprising tetravalent metal oxide/organic acid complexes, in particular those of cerium oxide, and, more especially, to such organic sols having great stability and controlled particle size.
2. Description of the Prior Art
Colloidal suspensions in organic media of micrometric or submicronic particles are known to this art, the properties of these colloidal suspensions being similar to those of pure solutions. These colloidal suspensions are generally designated by the term "sols."
These sols had the property either of being stable with time and being of coarse particle size (in general, hydrodynamic diameter on the order of one micrometer and high viscosity), or being of fine particle size but of low stability: half-life duration at most equal to one or two months, and of relatively low concentration. However, to date, as far as is known these properties have not been simultaneously obtained.
For certain applications, the aforesaid properties are essential. More particularly for the reasons hereinbelow outlined, the use of the sols as additives for fuels for internal combustion engines requires both the option of existing in a high concentration and the necessity of having a low particle size and a very high stability.
The use of the sols as additives for diesel engines is a representative example of the constraints to which such a sol must comply.
During the burning of diesel fuel in diesel engines, the carbon-containing products of combustion have a tendency to form soots which are considered harmful both to the environment and to human health. It has long been sought to reduce the emission of these carbon-containing particles, hereinafter sometimes referred to simply as "soots." The research in this area takes account of the necessity of not increasing the emission of carbon monoxide and of gases which are considered toxic and mutagenic, such as nitrogen oxides.
A great many solutions have been proposed for reducing these carbon-containing emissions.
However, attention is increasingly focused on fitting the exhaust systems of internal combustion engines with a filter capable of trapping all, or a very high proportion (at least 80% by mass) of the carbon-containing particles generated by the combustion of the various fuels. This technique is, however, limited by the storage capacity of the filter (or trap); it is necessary either to empty the filter or to incinerate the soots contained therein. This operation, known as regeneration, is extremely expensive and difficult to implement. One of the most commonly proposed solutions is the combustion of these soots, which combustion is intermittently carried out either by electrical heating or by use of a fossil igniter fuel.
Nonetheless, this technique presents many disadvantages, not the least of which is the risk of thermal shock leading to fracturing or cracking of the ceramic filter, or to melting of the metal filter.
One solution which would be satisfactory entails introducing catalysts into the soots which make possible frequent self-ignition of the soots collected in the filter. To accomplish this, it is necessary for these soots to have a self-ignition temperature which is sufficiently low to be frequently attained during normal operation of the engine.